1. Field of the Invention
This invention relates to a method and apparatus for measuring phase levels, interfaces and compositions in petroleum desalters and dehydrators by using sampling means in the unit to withdraw liquid samples from a plurality of known vertical levels inside the unit and passing each sample outside where an electrical property of each sample is measured which determines the water content of the sample.
2. Background of the Disclosure
Crude petroleum contains impurities which include water, salts in solution and solid particulate matter that will corrode and build up solid deposits in refinery units. These impurities must therefore be removed from the crude oil before the oil can be processed in a refinery. The impurities are removed from the crude oil by a process known as "desairing", in which hot crude oil is mixed with water and a suitable emulsifying agent to form a water-in-oil emulsion which provides intimate contact between the oil and water. The salts pass into solution in the water. The emulsion is then passed into a high voltage electrostatic field inside a closed vessel known as a desalter. The electrostatic field coalesces and breaks the emulsion into an oil continuous phase and a water continuous phase. The oil continuous phase rises to the top to form the upper layer in the desalter from where it is continuously drawn off and the water continuous phase (commonly called "brine") sinks to the bottom from where it is continuously removed. Similar equipment (or units) and procedures, except for the addition of water to the oil, are used in oil producing fields to dehydrate the oil before it is transported to a refinery. These units are referred to as electrostatic treaters, dehydrators and precipitators. This is because all crude oil pumped up from wells contains water and buyers do not want to pay for transporting, storing and pumping water or overloading desalters at the refineries.
During operation of such units, an emulsion phase of variable composition and thickness exists at the interface of the oil continuous phase and the water continuous phase in the unit. If the emulsion phase gets too thick, the oil continuous phase will contain too much brine and the lower brine phase will contain unacceptable amounts of oil. In extreme oases it results in emulsion being withdrawn from the top or bottom of the unit. Oil entrainment in the water phase is a serious problem as it is environmentally impermissible and very expensive to remedy outside the unit. Also, in many units it is desirable for the water continuous or brine phase to be as close as possible to the high voltage electrodes in the unit without resulting in shorting across the oil to the water, in order to achieve maximum coalescence of any remaining oil droplets entrained in the water continuous phase and thereby ensure that the withdrawn water phase is substantially oil free. If the emulsion phase gets too thick a demulsifying agent must be added. If the water continuous phase gets too high or too low, the water phase withdrawal valve at the bottom of the unit called a "dump valve" must be correspondingly opened or closed to the degree necessary to reposition the water phase to the desired level in the unit, and/or the oil withdrawal valve at the top of the unit correspondingly adjusted. Therefore it is necessary to monitor the level and condition of the phases in the unit.
This has traditionally been done manually by operators periodically opening trycock valves to withdraw samples from fixed levels inside the desalter. A trycock valve is merely a sample line comprising a pipe located at a fixed level in the unit, which is connected to a valve outside the unit. Some desalters have what is called a "swing arm" sample line in the unit in place of, or in addition to, trycock valves. A swing arm sample line is a pipe inside the unit open at one end for withdrawing samples, with the vertical position of the opening inside the unit manually adjusted by suitable means, such as a crank and gear mechanism outside the unit. In either case, an operator opens a sample valve to withdraw a sample and runs it over a smooth surface such as metal to visually determine if the withdrawn phase is oil or water continuous or if it is a stable emulsion phase. No quantitative information is available using this method and, further, because desalters typically operate at inside temperatures and pressures ranging between about 200.degree. to 300.degree. F. and 100 to 250 psi (dehydrators typically run at lower temperatures and pressures), there is a danger of the sample flashing and burning the operator. Also, the withdrawn sample may be different in phase identity at the reduced temperature and pressure outside the unit than it is inside the unit. Other methods have included the use of a float level to indicate the level of the water phase and also the use of a capacitance probe, some of which can give information about the water content of an oil phase, while others merely indicate if the phase is oil or water continuous.
Newer techniques have been introduced which employ means permanently mounted at a fixed position inside the unit for measuring an electrical property, such as energy absorption, of the fluid inside the unit. Some of these electrical means employ a probe which is able to determine the water concentration in the liquid phase surrounding the probe. In order to employ such devices in the desalter, the desalter must be drained, opened up and special nozzles and other devices welded to it. The welds require expensive post weld heat treatment. Moreover, it is simply not possible to shut down an entire refinery to modify a desalter. Consequently, any modifications must wait for a refinery turn around which can take five years or more. Furthermore, welding the devices at fixed positions on the desalter provides data only at those positions. Economic considerations prohibit the use of sufficient probes to obtain a profile of the conditions inside the desalter. Typically a probe is installed at the vertical mid-position of the drum and sometimes at an angle to try to obtain information around the mid point. Optionally, a second probe is installed near the oil outlet at the top of the unit as an emulsion carry-over alarm and a third at the brine outlet on the bottom as an oil carry-under alarm. The desalter is controlled by trying to maintain a certain water concentration at the single mid-level probe, with the outlet probes expected to serve as carry-over alarms.